


ask me for the moon

by fiveamradio



Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Accidental Voyeurism, Blow Jobs, Body Worship, Daddy Kink, Developing Relationship, Emotional Sex, Explicit Sexual Content, Father/Son Incest, First Kiss, First Time, Jealousy, M/M, Masturbation, Mind Games, Mutual Pining, Parent/Child Incest, Possessive Behavior, Praise Kink, Unplanned Pregnancy, bear with me here, very subtle implied abortion
Language: English
Status: Completed
Published: 2019-12-05
Updated: 2019-12-05
Packaged: 2021-02-26 00:00:45
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 9,409
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21684142
Author URL: https://archiveofourown.org/users/fiveamradio/pseuds/fiveamradio
Summary: "I don't want you to fuck Asagiri Gen again," Senku says, and his voice is harder than Byakuya has ever heard from him. "I don't want you to have sex with him anymore.""Why not?" he asks, bone-deep terrified of hearing the answer and desperate to have it at last.Senku has no such fears."Because it's not supposed to be him," he whispers, moving closer, lips skimming the delicate skin over Byakuya's wild pulse point. He's arching his back slightly, breathing in the fever-hot air between them. "It's not supposed to be anyone."Byakuya tightens his hold around Senku's beautiful waist. "Who is it supposed to be?"Senku lifts his head then, and stares straight at Byakuya. His eyes are hard, shining like vivid crystals, like stars have been set to glow on his face. "Me," he says.
Relationships: Ishigami Byakuya/Asagiri Gen, Ishigami Byakuya/Ishigami Senkuu
Kudos: 67





	ask me for the moon

**Author's Note:**

> mind the tags and bear with me.

The first time, it’s during class. 

Regular school classes bore Senku to death more often than not. Byakuya has been receiving complaints about his son’s behavior as a student since he was a child, about his unwillingness to cooperate, his tendency to point out mistakes, his amazingly fidgety attention span. Senku's mind works in an entirely different way than that of other people's, and Byakuya has known this for years, maybe ever since the first time he held him in his arms and saw his big, shiny eyes curiously peeking at the world around him. Senku is smarter, so much smarter - he's the closest thing Byakuya has ever met to a genius, a real genius, someone who was handmade by the universe so they'll unravel its secrets. And people have advised him before to put Senku in an advanced class, to speed up his education process at the very least; but Byakuya wants Senku to have as normal a childhood as possible before he sets off to change the world. He wants him to have friends his age, and parties to go to, and normal things to worry about. It's a healthy decision as a father, he thinks, but also means he needs to keep Senku's mind constantly stimulated and challenged, which makes him take his 12-year old son to his university-level classes twice a month. 

This, too, came from a challenge; sometime after his tenth birthday, Senku demanded to be taken, and Byakuya half-joked, half-dared him to get a grip on quantum physics if he wanted to - and Senku arrived months later with complete knowledge of the subject and a stubborn little pout, and Byakuya had no other choice. 

If it were up to Senku, Byakuya knows, he would simply stop going to his regular school altogether and spend all his mornings and afternoons trailing his father around in the Science Department of the university he works at; and Byakuya normally spoils his son rotten and gives him everything that is in his power to give, but in this he refused to bend, so they had to reach a compromise. Every other week it is, an agreement reached after a lot of Senku's stubborn silent treatment before he decided to let go, and that Byakuya had to go to his school to talk about so he wouldn't be riddled with too many absences. 

It works, nicely enough, and there's a particular delight for him in taking his son to work, having him in his small handmade white coat walking by his side and sitting on a desk and paying attention to what Byakuya is teaching, taking notes and asking questions and doing his best to embarrass him in front of all his students. It's thrilling, and it forces him to stay on his toes, keep himself updated and knowledgeable enough that Senku won't catch him on a mistake. 

It's routine, the type of day-to-day occurrence made better and more exciting by Senku's simple presence, so much so that Byakuya is astounded that he didn't notice this particular aspect first. 

He has no idea how he's never noticed his son's jealousy before, but now that he's seen it, he can't stop paying attention to it. 

Senku isn't touchy or conventionally affectionate by nature, or even by practice; he detests useless skin-to-skin contact, has always been painfully averse to physical touches, and he dodges from Byakuya's usual attempts to hug him or hold him. On a good day, he'll allow a hug before heading out to school, but most of the time he'll scoff at anything bigger than a head pat - and yet, whenever he's accompanying Byakuya at the university, he'll let him hold his hand and hug him and ruffle his hair. On one memorable occasion, he demanded a piggyback ride back to the bus station. 

Too happy with this change, Byakuya never took the time to wonder why that was so. And he's finishing up class and helping one of his students with a tricky part of an essay, when they touch two fingers to his arm, and Senku  _ tenses up _ by his side. 

_ Oh _ , he thinks idly, and shifts to the side just a little, closer to Senku and a bit further away from his student, and that seems to relax Senku a bit, enough to unclench his jaw and drop his shoulders. 

He's done with his explanation and waving the student away when he feels a small tug to the pocket of his coat, and he can't bite back his smile when he glances down at his son huddled close to him with a frown between his brows. 

"Hey, there," he says. "What is it? Wanna go home?"

Senku shakes his head, mouth set in a stubborn line. "Let's go to the Physics Lab," he demands, fingers tightening around the fabric of Byakuya's white coat. "I need to do some readings for a project and I don't have the tools for it at home."

"Aw," Byakuya coos, indulging in picking him up and making him sit down on the desk now that he knows Senku won't bat his hands away. "The Physics Lab is for masters-level projects, though," he teases, hands on the table at either side of Senku's crossed legs. "Are your blueprints that complicated already?"

Senku scoffs, rolls his eyes. "Of course they are. What the hell do you take me for?"

"My little genius," Byakuya says, pride bursting up inside his ribcage like a star. "We can go to the Lab today for a little while. Give me a list of whatever it is you need, too. I'll buy it for you."

Senku doesn't smile, not particularly, but his eyes shine a little in excitement, and that's all Byakuya needs. 

When they leave the classroom, Senku tucks his hand in Byakuya's. 

-

They live by themselves. 

Byakuya made a point of this when Senku was born. He himself is easily sociable, and he knows, communication skills born from a natural ability to maintain a conversation and from his years of experience as a teacher, as working in a position where he must learn how to make himself comprehensible for a large number of strangers. He has friends, a good relationship with his coworkers, and he could spend most of his time surrounded by people if he so desired. But he values his alone time and, more than that, he values Senku’s. 

They are very alike in many ways, Byakuya knows. From their obviously similar appearances, the same sharp red eyes and oddly colored hair, the same smirk and the same arrogant way of tilting their heads and looking down at someone when disregarding an opinion or a particularly stupid remark. Byakuya has always considered Senku the far better part of him - the smarter and quicker, the genius remaking of a more or less ordinary existence. There’s no jealousy in his comprehension of that; Senku is his son, the only person in the world he genuinely and desperately loves, and will grow and live to do things that far surpass Byakuya's most daring imagination. 

But that does not mean he can’t see the less than flawless aspects of Senku’s personality - one of them being his inability to properly interact with other human beings. 

Senku has had science as his first love affair and his first friend since the moment he opened his eyes to the universe. Senku's eyes are wide open and clear as an entire galaxy, his imagination bursting like a supernova. There's no type of social interactions that will ever be enough for a mind such as his; most of the time, human motivations go way past his comprehension, if he cannot put rational thought to them. More often than not, he simply doesn't care enough about them, preferring to be left alone in his laboratory with his projects and blueprints. And Byakuya has always spoiled him rotten, has always cared for him even when Senku's wishes go against Byakuya's own - and that is why Senku spends most of his time by himself when he's at home, tinkering with his ideas until he feels like indulging Byakuya with his attention every now and then. 

Byakuya does his best not to bother. Senku will come after him when he wants to, when he feels comfortable doing so, and Byakuya prioritizes Senku's comfort and ability to figure out what he wants for himself over any petty paternal needs for reassurance. 

But he  _ is  _ a father, in the end, and some habits are hard to get rid of, which is why he's standing outside his son's bedroom door, just to check if he's gone to bed already or, if he hasn't, to ask if he wants anything to eat or if he needs something for tomorrow. 

He only hears the soft moaning when he has his hand wrapped around the handle. 

It's so soft he almost misses it; but it's there, gentle and whispered through the air, the small sounds coming from inside Senku's bedroom, and Byakuya has always been too rational to ignore a logical conclusion when he sees one. 

He tells himself it's curiosity that makes him linger, pure, unbridled sense of wondering that keeps him standing, one hand on the metal handle, staring at the wooden door and listening to the sounds of his 14-year old son masturbating. 

There's something peculiar about this, about this knowledge being presented directly at Byakuya like the obvious result to a chemical reaction. Senku is 14 now, and it'd be perfectly expected of him to take care of whatever sexual urges he may have. But Byakuya has grown used to seeing his son peeking at him from behind a telescope, scoffing at him over a computer screen, tinkering carefully with overly complicated projects and sketching intricate, detailed blueprints. Senku has always been the image of perfection, of flawless, beautiful efficiency, so much so that if Byakuya hadn't been the one to give birth to him, if he hadn't been the one to hold him in his arms and shush him when he cried and rock him to sleep, the one to patch up his skinned knees and kiss them all better, he would think his son hardly human, hardly made of flesh and blood. It's easy to forget Senku is just a fourteen-year-old boy - and this is why it's such a surprise to catch him doing something as ordinary as masturbating. Byakuya wonders what he thinks about when he does it, who he thinks about; wonders if his urges are those of normal people's, or something else entirely, something different and unique to match the special glow of his mind. In a strange middle-of-the-night haze, Byakuya wonders if Senku thinks about him, as he thinks about Senku - but it barely lasts a second, lost to the sultry, comfortable darkness and its fog of thoughts. 

Senku is quiet, he realizes idly, so  _ quiet _ . There's very little to listen to aside from the occasional rustling of sheets and one or two sharp intakes of breath - and it's over quickly, one louder, shuddering sigh and then nothing more, just what sounds like Senku shifting on his side and getting ready to fall asleep properly. 

He leaves easily, after that; and when he lies down on his own bed, one hallway-length away from his son, he doesn't touch himself or has his thoughts wander into darker paths. Senku's satisfaction after his orgasm feels like Byakuya's own, and he falls asleep peacefully, as if Senku's pleasure trickled over to rest just under his skin. 

-

Senku weighs nothing in his arms. 

It's a common occurrence for Senku to lose track of time and fall asleep on his laboratory, collapse from exhaustion atop his blueprints and his circuits and projects; and Byakuya has grown used to picking him up and cradling him close to his chest to tuck him in bed, fold all his blueprints properly and close up the programs and place all his things back where they should be, so that none of it is damaged or lost, so that everything can be picked back up the next day. It's the type of care Senku doesn't deny him, the type of mindless comfort Byakuya loves doing for him - his son has never been a particularly clingy child, but he has always been slightly affection-starved, and even if he constantly dodges from Byakuya's attempts to touch him, when he's tired or sleepy he tends to lean into them, too exhausted to fight back his inherent need for softness. There's something precious for Byakuya in the way Senku nuzzles his neck when he's being carried to bed, in feeling his heartbeat fluttering against his chest. 

He's doing so right now, shutting the door to the laboratory close behind himself with his foot while he keeps Senku in his arms - and then Senku squirms a little in his sleep, lets out a small whine, and hides his face on Byakuya's shoulder, and something  _ shifts.  _ Twists around on itself, just slightly, just  _ enough _ . 

It's not the first time Byakuya has noticed how beautiful his son is, but it is the first time he realizes what exactly that implies. 

Before, he's always comprehended Senku's beauty as a theoretical fact, something he could understand from a safe distance. One more little dot on the list of things about him. Senku has red eyes, white hair with green tips, a tendency to scoff when something isn't to his liking, a preference for mechanical pencils over pens. Senku is smart, left-handed, fond of energetic drinks,  _ pretty _ . 

But now, as he places him on the mattress and tucks him under the galaxy-patterned blankets Byakuya bought for him when he was a child and that Senku never quite learned how to let go of, he realizes just how beautiful he is. How close his hands are to Senku's gorgeous face, the soft part of his lips, the warmth of his skin. For the first time, he realizes he could touch, he could feel; there's a real distance between their bodies that could be crossed, if only Byakuya leaned down, if only he moved closer,  _ if only _ -

His mind is feverish, his brain hazy aside from its sharp focus on Senku's sleeping form, huddled close to the blanket. It's the first time, he notices, in a disturbing daze, that he's noticed his son as someone desirable. 

He ought to step back, he knows, ought to leave and turn his back and go to his own room, and never spend a single second thinking about what he's realized today. But he finds himself sitting by the edge of the bed, folding his hands on his lap as a way of stopping his body from reaching, from doing things he will not be able to take back if he ever puts them to reality, and he  _ watches _ . 

He doesn't do anything aside from that. He barely even moves. He sits by Senku's feet, and his eyes slowly get used to the darkness, and like this he can see the way the blanket dips down where it's covering the sinuous curve of Senku's waist, the gorgeous line of his hips. There's a simple, raw delight in  _ seeing _ . Senku is wearing very little, since he prefers to stay comfortable when he's at home, and the covers are sliding off from his body, his thighs naked and so beautiful Byakuya can feel his mouth water with the need to press his lips to them. If this was happening during the day, he feels, he would tear himself apart with guilt, would drive himself insane with shame; but like this, he's living on a strange, eerily comforting dimension, where he can let his thoughts swim away from whatever morning-light restraints he keeps them behind most of the time, where he can  _ desire _ his son. 

He barely knows what is it he's yearning for. He wouldn't be able to touch Senku right now even if he wanted to; Senku is too perfect, too impossibly flawless for the likes of him. His fingertips are tingling with stardust, however, desiring to pour all of his devotion on the dip of Senku's collarbone, to melt his existence all over Senku's body. For a feverish moment, Byakuya has a realization that this is what he was born to do: he was born to take care of his boy, to devote himself to him and worship him, unmake himself with adoration for him. Like this, his rawest bodily desires feel natural, a simple extension of the desperate love he has always kept for his son, as if he's yearning for him with his body alongside his mind and heart. Everything that he is. 

Senku shifts and turns a little, presses a small sigh to the softness of his pillow, almost as if he's reacting to the burning heat of his father's gaze caressing over him. It's such an endearing, pretty sight that Byakuya can't help himself - he leans forward, presses a desperately devoted kiss to his temple, galaxies trickling from between his lips. 

Senku smiles in his sleep. 

-

Byakuya doesn’t trust Asagiri Gen. 

That's the first thing he decides about the man standing on a pristine suit at the door of the auditorium, all pretty smiles and perfect handshakes. 

He learned Gen was giving a lecture earlier this morning at the teacher's room over at the Aerospace Science department - one of his coworkers commented that they were dismissing class for the remainder of the day so they could catch the presentation. Byakuya was astounded; he'd heard of Asagiri Gen, as everyone with any sort of telecommunication device had. But he'd heard of him as what he spent most of his time doing: self-proclaimed mentalist, a con artist at the end of it, someone who made use of humanity's weakest, most fragile aspects to draw money out of them. There was a perversity to him, Byakuya had always thought, something insidious hiding in the corner of his lips and his breast pockets. To find out someone like him was being invited for the utmost honor of giving a lecture in such a prestigious university was a surprise, mostly unpleasant. 

He didn't dismiss his own class for it, and decided to follow through with his planned schedule for the day. He had better things to do than sit on a chair and listen to Asagiri Gen's viciously sweet voice drawling on a microphone about god knows what. Still, there was a strangely electric feel to the air, something slightly uncomfortable whenever Byakuya moved or turned to speak to his students, as if there was a pair of eyes looking at him when he had his back to them. A constant shadow on the peripheral edge of his vision, something that has lingered in atop his shoulder and to the back of his skull ever since he caught Senku in his room, and that grows heavier and sharper the more time he spends in Gen's general vicinity - until, by the time he finishes class and starts packing to leave for home, his skin is crawling with it, his nerves slightly frayed with discomfort. 

If Senku were here, he figures, he'd call him a stupid old man and tell him to get himself together. The thought makes him smile, and it's almost enough to make his tensed up shoulders relax and his jaw unclench; but then he passes by the auditorium, and he sees Gen, and his entire body reacts to it as it would to a threat - he can feel himself tensing up, can feel his back straightening and his eyes narrowing; and Gen crosses glances with him, and  _ smiles _ . 

Byakuya is thankful that the lecture is over. He can push through the mass of people leaving the place and avoid any extended contact with any of them, take his usual way to the bus station, and then rest until he's arrived home. It's as he's clutching his bag closer to himself and reaching inside his pocket to text Senku and tell him he's on his way that he notices it's raining; a dreadful storm, unlike what would be expected for this time of year, and he's barely had time to think about what that means when something slides up to press too close to his back for comfort and a purred voice whispers, "Dreadful weather."

He startles, even if he doesn't mean to. The voice is too close, the warmth of another person's body too near; and then he twists, and Asagiri Gen's uncannily pretty face is inches from his own. 

"Yeah," he says, takes a step back to put some distance between them. Senku has taught him the precise distance to maintain between himself and other people in order to avoid the need for a handshake or, worst, a greeting hug, and that is what he attempts to do, almost unconsciously - but Gen is smoother than he is, and steps forward just as he's fitting his foot to the floor behind himself. "Terrible storm," he continues, unsure of what he's meant to do in such a situation. 

"I'm glad I got a parking spot close by," Gen comments, voice easy with casual conversation, as if they're acquaintances, as if it's perfectly normal for both of them to be talking about the weather even if they've never seen each other face to face before. His tone is even sultrier in person, the words almost slurring together with how beautifully he drawls them into each other. "I'd hate to walk all the way to my car in this rain." He tilts his head to the side then, an angle meant to be natural but that ends up lingering just on the curve of uncanny. "What about you, professor Ishigami?"

Byakuya has a chilling moment where he has no idea of how Gen figured out his name or the fact that he's a teacher - then he remembers his ID card, pinned to the pocket of his shirt, and the student essays he's carrying, and it helps him relax. But still, it means Gen is more perceptive than Byakuya gave him credit for, or at least perceptive enough to take a piece of learned information quickly into conversation, and that already means a depth of intelligence and fast reaction Byakuya did not expect from him. 

"I take a bus home, unfortunately," he says, seeing no reason or way to lie. "Guess I'll wait until it lets down a little."

Gen hums, a little note of consideration. “I could give you a ride," he offers, tone so light Byakuya is drawn to comprehend that as a usual addition to his day, as if this has been happening for years, and not something being suggested to him by essentially a virtual strange. "To the station at the very least.” 

Byakuya swallows dry, glances down at Gen. The height difference is there, and obvious, but Gen is staring at him with absolutely no inhibition in his dangerous dark gaze. "I would hate to be a bother," Byakuya answers, and Gen smiles wider. 

"Why, it's hardly a bother," he purrs, appearing extremely satisfied by the turn of the conversation. "I'll go through there on my own way home. Please, allow me." 

Byakuya realizes, with a startle, the situation he's been gently coaxed into; there's no polite way to refuse the offer, and they're not intimate enough for Byakuya to be rude to him without breaking the fragile social rules. It would be irrational to say no, considering he really has no better way to get home, and there's no decent explanation to use. He tightens his hold around the strap of his bag, and Gen keeps looking up at him expectantly, a pleased cat who got his prey right where it wants. 

And, to make it worse, Byakuya has no idea what Gen wants from him. 

"Well, if you're sure it won't be a bother," he says in the end, relenting. Surely, he thinks, he can survive a car ride with a 20-year old brat. 

"Brilliant." Gen produces a bundle of keys seemingly from out of nowhere, twirls them around his finger. "Shall we?"

At least, Gen wasn't lying - his car  _ is  _ parked very close by. It's a discreetly expensive vehicle, the type of which declares Gen's personal elegant taste. There's nothing tacky or extravagant about it, as Byakuya would have expected, but it's obviously costed a lot of money, enough to make him slightly uncomfortable but not enough to actually call out on. 

"I didn't see you at the lecture," Gen says as he's pressing the button to start up the car.

"There were many people," Byakuya answers, keeping his bag over his lap, clutching to it tight. "How would you even know?"

Gen giggles, a distinctly pretty sound. "I have a good memory for faces, sensei." He looks Byakuya up and down, licks the corner of his lips. "I'd have remembered yours."

"Yeah, I didn't go," Byakuya says, unwilling to caress this boy's ego. "I'm an aerospace engineer, psychology's not really my thing."

Gen nods. "Understandable. A lot of people came because they knew my name, honestly. My masters tends to be boring for people outside of my area of study. But I won't pass up an opportunity to talk without being interrupted, and it's nice to speak about things other than the usual stuff I go on and on about in talk shows."

"So, you have a degree," Byakuya says, slightly taken aback. " _ And _ a masters."

"And I'm working on a doctorate, yes." There's no arrogance in Gen's voice when he states so; it appears a mere declaration of fact, as if there's nothing to be celebrated for in that. As if he's not a 20-year old halfway through his doctorate. 

"Why do you work as what basically comprises of being a con artist, then?"

Gen's laughter then is maniacal, verging on delighted hysteria. It drags at some unpleasant edge, carries something heavier than it appears to. " _ Oh _ , sensei. Being  _ basically a con artist _ gives me so much money you wouldn't believe." He taps on the steering wheel in a strange little pattern that almost looks like Morse code, moves his hands up and down idly. He has beautiful fingers, Byakuya notices. Long and elegant, not a single scar or mark. A doll's fingers. "I'm a shallow man with shallow desires. I have no particular love for science, or willpower to help humanity evolve. I'm no such idealist."

Byakuya thinks about it as he stares at the heavy rain slamming down on the window. "Why the academic work? If you're uninterested in science."

Gen hums, sounding pleased. "Sharp mind, as would be expected. Well," he says, shifts gears in a smooth gesture. "I like learning things. And the most natural step after graduating from college was a masters, and then a doctorate, so here I am. Talk shows pay a lot, but don't demand much brain effort. I'm curious by nature, I suppose."

Then he tilts his head to the side, leers at Byakuya's ID card. "As I'm sure you noticed."

"You sure notice a lot," Byakuya comments, shifting a little on his seat. 

Gen shrugs. “Comes with the job. I get paid to notice things people usually don't. Speaking of which, you have a son, have you not?”

Byakuya shivers. He can't help it. There's something vicious in the way Gen says it, something that speaks of depths Byakuya is unwilling and unprepared to handle. "Yeah," he answers in the end, because he has no other choice other than allow himself to be coaxed and dragged into the conversation. "How do you know?"

Gen gestures vaguely with his head. "Your lockscreen. On your phone. As I said, I get paid to notice things. You're rather young to have a son of that age, sensei. How old is he, 13? 14?"

"He's turning 15 next month."

"How  _ sweet _ ," Gen drawls, sounding almost predatory. "You must have had him rather early, since I assume you have to be on your early thirties, at  _ most _ ."

Byakuya scratches the back of his head, and finds himself blushing even if he doesn't want to. "Not that young, really. I had him in my twenties."

"It is still rather young, though. You had him close to your graduation, did you not?"

Byakuya takes a deep breath. “Senku was… an accident.”

He still remembers, the pregnancy test, the way his legs gave out under him, the way his mind went blank with desperation so complete he had no way of making a single coherent thought. An accident, maybe, a slip, a cruel sleight of hand, and Byakuya suffered more than he ever thought possible; and then he held Senku in his arms the first time, and Senku blinked up at him with impossibly clear and beautiful eyes, and Byakuya realized nothing in his life would ever make him as happy as the little boy pressed close to his chest. 

" _ Senku _ , huh?" Gen says. He savors the name, it seems, like a connoisseur would savor a particularly pleasant sip of an expensive wine. "A thousand skies. Beautiful name."

“I had one of such...ah,  _ accidents _ in college as well," he continues, as easily as he started. "But I dealt with mine in an entirely different way. I take fate was kind to you, though - he looks like a wonderful kid.”

“He’s everything to me," Byakuya blurts out, barely rational about it, the same as he's barely rational about anything when it comes to his son. "The best thing in my life.” 

"I'm assuming he must be smart as well."

"He's - a genius, honestly," Byakuya admits, mind still hazy from the way the conversation is progressing. "The smartest person I know. He's had offers from a bunch of universities for early enrollment, but I wanted him to have a more or less normal childhood."

Gen smiles, and for the first time since Byakuya laid eyes on him, it actually almost looks genuine. "Good idea. I enrolled early at my own college, and I'm still not sure that was the right decision. Prodigies tend to fare badly in the real world. Not that my father cared much either way." The last sentence flies by like a fluttering breeze, so naturally and quickly Byakuya barely has the time to properly analyze it as the confession it is. "Are you and Senku very close?"

Byakuya is caught off guard again, unsure of how to answer. Not for the first time, he's dragged immediately back to himself standing outside his son's door, listening to the small sounds of him - "I suppose so. It's just the two of us."

"He's a beautiful boy, you know," Gen comments, such a delicate and wispy statement of something Byakuya has been tearing himself to shreds to avoid thinking about for the past weeks. "Looks like you. I'm glad he has you."

Byakuya is still thinking this over when they reach the station - and then Gen is a flurry of goodbyes and vague, swooping gestures, and Byakuya is leaving the car before he can properly catch the thread of whatever it was Gen left implicit in his last sentence.

"Give Senku-chan my regards," Gen says, and smiles, and drives away; and Byakuya is left alone on the bus station, rain falling hard on the sidewalk and too many thoughts in his head for comfort. 

-

Senku finds the slip of paper with Gen's number inside Byakuya's breast pocket before Byakuya himself does. 

He steps forward as soon as Byakuya gets inside, pushes up far too close into his personal space and fits his fingers to his pocket, pulls out a small paper in a dangerously smart move. 

"What's this?" he asks, a bloody edge of curiosity in his tone, turning it this way and that. 

"Who knows?" Byakuya says, a strange tingling on the bottom of his spine. He feels analyzed by that sharp gaze, feels as if he's been stripped bare to his bones and organs under Senku's impossible intelligence. "You found it before I did. You tell me."

Senku lifts it up against the light, stares at it with a lovely frown. "Asagiri Gen," he reads, the syllables of Gen's name curling and smoothing in his voice. "He left his number." He scoffs, folds the paper and tucks it gingerly back inside Byakuya's pocket. "I thought you'd have better taste than that, old man. Although he's not that bad-looking, I suppose. Even if he's, like, half your age. Isn't he closer to  _ my _ age than yours?"

Byakuya's face flushes red and hot with embarrassment. "I never said I did anything with him," he points out, unwilling to look into Senku's eyes. "He was giving a lecture at the university and offered me a ride to the station. I didn't even notice he'd given me his number."

Senku laughs, mocking. "Sleight of hand. Fits the magician." He turns his back on Byakuya, giving him an idle wave. "There's leftovers from last night, unless you're going to have dinner with your new mentalist boyfriend."

"Not my boyfriend," Byakuya says in protest, and Senku laughs again, as if he doesn't care either way, as he leaves for his laboratory; but his shoulders are tensed up under his shirt, and Byakuya can read discomfort in his son clear as day, and it's obvious he's more than a little displeased with the outcoming of the situation. 

Aside from that, the night progresses uneventfully. Byakuya isn't sure what to do with the slip of paper inside his pocket, so he takes it out and sets it down on his bedside table, far from Senku's sight so it won't make him mad anymore, and goes back to work to stop thinking about it, grading papers he needs to get done by the end of the following week. It's mechanical, mostly brainless work, and when he gets up to make dinner for both of them, it's late, but not late enough that Senku will be asleep, considering the boy's disastrously messy sleep schedule. Byakuya debates against going to knock on his laboratory's door to bother him and ask if he wants to eat - but something inside him is yearning for, if nothing else, a glance at his son's face, just reassurance of his existence. 

When he hears the soft gasping, he  _ freezes _ . 

Ever since the first time he caught Senku masturbating, he's avoided heading to his room too late at night, unwilling to think too hard about what happened, about what he felt; but it's not that late, not late enough, and Senku is still in his laboratory, which  _ means _ \- 

A whispered moan, a gasp, and Senku's voice breaking on, "Daddy,  _ please _ ."

Byakuya has to clasp his hand over his own mouth so his throat won't betray his position, has to square his feet on the floor so he won't be pushed by his own bodily instincts to go inside, to go after Senku, to -

His mind is spinning, his vision hazy; on the other side of the door, Senku is whining quietly, undeniable sounds of him touching himself quickly and hastily, and with every other breath he whimpers, "Daddy," and "please", and he sounds so much prettier than Byakuya ever imagined in his wildest, darkest, most hidden dreams; he sounds perfect, probably  _ looks _ perfect, sitting on the chair Byakuya bought for him, legs spread and white coat half-falling from his body, beautiful hand moving in stuttered movements, hips bucking up and his bottom lip snagged between his teeth. If Byakuya closes his eyes and focuses, he can almost  _ see _ it, can almost taste the feeling of Senku's skin on top of his tongue. 

He's fitting his hand under the waistband of his jeans before he realizes it, pushing the tips of his fingers hard against his clit with merciless pressure. He shudders, full-body reaction, and on the other side Senku  _ moans _ , as if he was the one to be touched, as if he  _ knows _ . 

-

He fucks Gen. 

It's the natural progression of things, so of course he does. Gen is attractive in whatever sense of the word, absurdly beautiful, so gorgeous that it's almost easy for Byakuya to ignore the fact that he's barely two years over legal. He's also  _ smart _ , dangerously so, able to keep fascinating conversations while also being just insidious enough for his words to constantly verge on triggering Byakuya's fight or flight response. It's exhilarating, having sex with him, spending time with him, and Byakuya finds himself making it into an almost routine. 

It's easy to take the subway to Gen's disgustingly, scarily expensive apartment complex -  _ I have so much money it'd make your skin crawl, sensei, so let me know if there's anything your boy wants as a gift - _ take the elevator up, knock because he hates the sound of Gen's stupid fancy doorbell, be greeted by a pretty fox-like smile and a pair of smooth arms wrapping around his neck to tug him inside as a sultry voice is already whispering into his ear, hands sneaking under his shirt and pulling his belt free from his pants. 

It's comfortable and  _ luxurious _ , almost. Gen is young but experienced, so skilled at what he does that it nearly makes Byakuya worried that someone of his age would be so practiced at everything they do - but Gen tenses up dreadfully at any mentions of his personal life aside from his job, and more so if any of them are related to his past. It drives his narrowed eyes into dangerously sharp blades, and Byakuya would rather not create any unnecessary tension between them. 

He does what he can to help; he makes decent homemade food when he thinks the house has gone too long without any proper meals, he washes the dishes, he holds Gen when they're sleeping together and his nightmares make him grit his teeth and cry in his sleep. It goes on for a handful of weeks, and it's  _ easy. _ Gen is beautiful, sex with him is fantastic, and he's good company more often than not - and the more he fucks Gen, the less time he spends agonizing over Senku, and he thinks they have a pretty good agreement going on, until one night Gen is fitting his robes back over his body and Byakuya is indulging in the simple delight of watching a gorgeous man, and Gen throws him a glance over his shoulder, smiles  _ awfully _ and says, "I can't be a replacement of Senku forever, sensei."

Byakuya has slapped him with the back of his hand before he can even fully process what Gen just said. 

"As if this changes anything," Gen laughs, appearing absolutely unmoved by the violence that just nearly knocked him backwards - he presses the pad of his thumb to the blood welling up from his wounded bottom lip, and licks the blood out of his skin like a kitten. "Just do what both you and the boy want. What's the  _ point _ of dragging this out?" he purrs, and lets his body fall back again on the bed, throwing his head back in some sort of sick, manic delight, laughing so beautifully it  _ destroys _ Byakuya inside and out. 

-

Byakuya arrives home in the worst mood he remembers ever being in. The world feels as if it's slipping between his fingers, crashing way beyond any sort of recovery - damn Gen to hell and back and there again, he thinks, curses, rages. Damn him for ever saying that, damn him for implying and bringing it to light, damn him for  _ knowing _ something that was supposed to stay trapped in the darkest corners of Byakuya's mind for the rest of his life. 

_ Just do what both you and the boy want _ , Gen drawls inside his ears, and Byakuya snarls at nothing. 

When he closes the door behind himself, Senku is sprawled on the couch, fingers quick as he plays one of his favorite games in one of his portable consoles. 

“What happened?” Senku asks, looking at Byakuya with a mildly curious look in his sharp red eyes as Byakuya starts rummaging the kitchen for something to eat. “You look pissed.”

“It’s fine.”

The curt answer makes Senku turn around completely, letting his game drop forgotten on the couch, one eyebrow raised in puzzlement. “Well, clearly it’s  _ not _ fine," he states, the charming all-knowing arrogance in his tone grating at Byakuya's nerves, "since you’re slamming the goddamn cupboards around.”

“I  _ said _ it’s fine!”

It’s snapped and mean, the type of mindless cruelty Senku has never been used to getting from his father; and Byakuya regrets it immediately when he sees the way Senku flinches away from him. 

“I’m sorry,” he mumbles, dreadful mood disappearing in a second at the idea of upsetting his son. He moves slowly, sits down next to Senku. “It’s just - I’m sorry.”

Senku says nothing, merely wraps his arms around himself. 

Someone once said prodigies' hearts are fragile like glass, easy to break. 

“C’mere,” Byakuya says, extending a hand. “Give this stupid old man a hug. I’m sorry for snapping at you.” 

After a moment of silence, Senku goes, head down and shoulders hunched. He allows Byakuya to hold him and shush him gently, press kisses to his forehead and apologize over and over again; but he's still upset when Byakuya peeks down at his face. 

"What is it?"

"You were with that guy, weren't you," Senku says, voice almost a whisper. "You were having sex with him."

Byakuya has never been able to lie to Senku. "Yes," he answers, rubbing his hand up and down Senku's back soothingly. "I was."

"How many times?" Senku asks. "How many times did you fuck him?"

Byakuya could say he doesn't remember, but he does. He remembers every single time he stepped into Gen's lavish bedroom, every time he untied the overly complicated laces tying his robes together and laid down on his huge bed and allowed him to touch. "Five times."

Senku nods. "Was it while I was at school?"

"Yes."

"In those days where you don't have classes in the morning. Tuesdays and Thursdays. Not more than once a day, or you wouldn't have been here when I got home from school, and you always were. He probably has a busy schedule too, so maybe not twice a day, but he was the one to propose you, so definitely every week. So if you met him every week, it's been over a month already, hasn't it?"

Byakuya lets out a humorless, sad chuckle. "That's right. You're so smart."

"Hm." Senku sniffs, rubs at his eyes with the back of his knuckles. "Where do you two meet?"

"At his house," Byakuya says. "His apartment. I didn't want you to see it, so I never brought him over." 

Senku nods again, buries his face once more in Byakuya's neck. His fingers keep fiddling with the hem of Byakuya's shirt. 

"Hey, dad," Senku starts. 

"What is it?"

"I don't want you to fuck Asagiri Gen again," he says, and his voice is harder than Byakuya has ever heard from him. "I don't want you to have sex with him anymore."

"Senku -"

"You always give me whatever I want, don't you?" Senku presses on, and his hands are tightening into fists at the front of Byakuya's shirt, dragging it slightly off-center. "So this is me being the damn spoiled brat you raised. I don't want you to meet him anymore. I don't want you to go over to his house. I don't want it."

"I won't. I won't if you don't want me to."

"Good. Don't."

Senku falls into silence once more, but seems unwilling to let go of Byakuya - like this, he has his thighs pressed close to Byakuya's hips, and he's shifting, and Byakuya can feel his body thrumming with misplaced desire, his mind going hazy with want. Senku is breathing warm on his neck, and he feels so  _ perfect _ . 

"Why not?" he asks, bone-deep terrified of hearing the answer and desperate to have it at last, admittance, the confirmation that he's not alone in this. As if he's stepping over the edge of a cliff, suspended on thin clouds, ready to fall. 

Senku has no such fears. 

"Because it's not supposed to be him," Senku whispers, moving closer, lips skimming the delicate skin over Byakuya's wild pulse point. He's arching his back slightly, breathing in the fever-hot air between them. "It's not supposed to be anyone."

Byakuya tightens his hold around Senku's beautiful waist. "Who is it supposed to be?"

Senku lifts his head then, and stares straight at Byakuya. His eyes are hard, shining like vivid crystals, like stars have been set to glow on his face. "Me," he says. 

Byakuya's entire body  _ shudders _ . "Senku," he gasps. "Senku, we can't -"

Senku wraps his arms around Byakuya's neck, and arches up, enough that their lips are almost touching, their breaths mingling together. It's so intoxicating Byakuya is dizzy, shivering, the edges of his perception going hazy with desperate desire. "Please." Senku's bottom lip is wobbling somewhat, and he sounds so needy he's almost upset about it, as if he has no idea of what to do with what he's feeling. "Please, dad. I know you - I know. I want. I want it too."

"Senku, we're -" 

Senku surges forward, and presses their lips together hastily, and Byakuya is kissing back immediately, almost as an instinctive reaction, as if his entire body is melting and tilting and shifting for Senku, Senku's touch, his existence. Their mouths slide, Senku presses closer, Byakuya holds him tighter, and they're kissing, the impossibility of the universe pooling atop their tongues, galaxies surging to wrap around both of them. Byakuya can't think, can barely make sense of who he is; 

When they part, Senku is breathing hard, the tip of his tongue darting to lick at his mouth as if savoring the lingering taste; and his lips are so  _ pretty _ , red and glistening wet, and Byakuya can't stop  _ looking _ at them.

“This was my first kiss,” Senku says, voice airy, floating on pleasure still, and Byakuya  _ shivers.  _

"God, Senku," Byakuya groans. “You can’t just  _ say  _ that.”

“But it’s true," Senku presses on, eyes focusing back again on Byakuya, painfully sharp, red like dripping blood. Byakuya could cut himself on Senku's gaze. "You were my first kiss. I was waiting for - for this. For you." 

Byakuya's head is spinning, his entire existence spiralling out of its axis. "Senku," he breathes out, dizzy. Somewhere in the back of his mind, there's some faint protest, some light remnant of what kept him from touching and reaching out for weeks, months, years. He can't remember, can't bring it up properly. It's hard to think why he should stop when Senku is perfect and warm and real against his body, on his lap, clinging to him with a desperate sort of yearning. "We shouldn't be doing this."

Senku scoffs disdainfully, and he sounds so much like himself that it relaxes Byakuya immediately with the soothing weight of familiarity. "What, are you scared? What are you scared of? Going to hell? You want this. I know you do. I see the way you look at me. Am I not more important than whatever stupid shitty morality you have going on in your big head?"

"Didn't you say," Senku continues, tilting his head again, leaning forward to skim their lips together, "you'd give me anything I wanted? I  _ want _ you."

Byakuya inhales a shuddering breath; his heart is beating into overdrive, straining against his ribs, pushing against his sternum. 

"I want you," Senku says again, " _ daddy _ ."

Byakuya is moving before he can quite register what he's going; but he's tightening his arms around Senku, capturing his gorgeous, smart, killer mouth in another kiss. A deeper one, this time, Byakuya's tongue pressing to coax Senku's lips into parting, until he's tasting his son completely, savoring and relishing in the small sounds he makes, the whined little notes of need and shattered moans. 

"Figures you'd have a thing for that," Senku teases when they part once more, but Byakuya can feel him shivering on top of his lap, and his pupils are blown wide, pitch black-hole dark in the middle of the blood-red of his irises. "Old man."

"You were the one who was moaning it against your pillow the other night,  _ boy _ ."

Senku blushes a deep, gorgeous pink. "So you  _ were _ listening."

"So you  _ were _ doing it for me."

"Shut up."

"Smart mouth," Byakuya says, and hosts Senku up as he gets to his feet, enjoys the little heated yelp of surprise as he fits his hands under Senku's thighs and has him wrap his legs around his hips so he can carry him to the bedroom. He's stepping towards Senku's when Senku lifts his face from where it's buried on the crook of his neck and says, "Yours."

Byakuya stalls in his movements immediately at Senku's demand. "What? Why?"

"Because every time I thought about it, I thought about doing it in your bedroom." He hides his face again, and Byakuya can feel his warm breath against his skin as he continues, "Your big bed and - and your smell everywhere and just. You."

It's enough to send a desperate piercing of heat down Byakuya's spine, enough to make him almost falter in his steps. "You're going to be the death of me," he whispers feverishly, and Senku shivers, and Byakuya is walking the rest of the way to his bedroom, pushing the door open with his foot and dropping Senku on the bed with all the gentleness he can muster, perfect care for his perfect boy. 

"Fuck," he gasps as he takes in the mental image of Senku, well-kissed, cheeks blushing like stardust, body shivering a little, sprawled atop his bed and his blankets, reaching for him. "I thought of this so many times."

Senku surges up, impatient and lightning quick as he's always been, wraps his arms around his neck and pulls him down. "Come make it real, then."

Byakuya is helplessly, painfully devoted, and he obeys, goes down when Senku brings him in, and kisses him again, and again, until he's nearly drunk with it, until his hands start moving. His body is shattering to crystal facets, his fingertips tingling; when he reaches under Senku's shirt, fits his palm to the smooth skin over his ribs, the hummingbird quickness of his heartbeat, they moan together, pleasure spreading between them, filling the very air they breathe. 

"Touch me," Senku says, demands, and Byakuya has always been so impossibly weak for his son's orders.

It is a type of wonder akin to the first time Byakuya saw the stars, to the sheer, desperate passion he feels for the cosmos. Touching Senku like this, with no restraints, no limitations, nothing to stop him aside from the boundaries of his own feverish imagination, feels like a revelation, like the realization of some need inside him that he never thought could grow quite this unbearable. It's worship, devotion at its finest, and he can't stop touching and kissing and feeling, as if he's rediscovering reality for the first time. He kisses the arrogant, lovely curve of Senku's jaw, the softness of his cheeks, the length of his neck all the way down to the riverbed of his collarbone, and he can feel galaxies on his lips, everywhere he touches, every little sound Senku makes. 

"So sensitive," Byakuya whispers, absolutely amazed, as he kisses down the sinuous curve of Senku's gorgeous waist. Goosebumps riddle Senku's skin at his light caresses. 

"Any complaints?" Senku says, sounding confrontational but almost embarrassed about it, and Byakuya laughs at the impossibility of the idea of ever finding anything to dislike about him. 

"You're perfect, baby," Byakuya breathes out as he rests his head over Senku's stomach, as he indulges in fitting his fingers to the softness of his quivering inner thighs. "You have no idea how perfect you are to me."

It's merely a statement of fact - but Senku shivers, a violin string being tugged, and whimpers, and his fingers wrap into fists around Byakuya's hair, and Byakuya inhales sharply. 

_ Oh. _

He takes a deep breath, pushes Senku's clothes out of the way completely, and he says, "You're the most beautiful boy in the world."

The words flow like pure, distilled sincerity from his lips; and Senku reacts to them immediately, so strongly Byakuya is sure he has the smallest control over his own body's responses right now. He keeps touching, keeps trailing kisses over Senku's shivering figure, coaxing sounds out of him, and keeps  _ talking _ . "You're the prettiest thing I've ever seen," he whispers. "The love of my life, my entire universe. You always do so well, you make me so proud, you're  _ wonderful _ -"

All of a sudden, Senku's hand flies out, grips Byakuya's wrist in a painful grip. When Byakuya lifts his gaze, Senku's eyes are shining with unshed tears, and his lip is red and swollen from biting, and he whimpers, "I - I can't - just -"

Byakuya shushes him like he used to do when he was a child, presses kisses to his temple and caresses his hair until Senku's breathing has leveled out to something resembling calm. "What is it?" he asks, softly. "What do you need, baby? My little genius, just ask me, I'll give you anything you want."

Senku shoves Byakuya's hand between his legs, cupping the hard weight of his cock, and he gasps out, " _ Please _ , dad, it  _ hurts _ -"

Byakuya goes, presses one last, lingering kiss to Senku's lips, and lowers himself down until he has his mouth inches away from where Senku needs it the most, and were it anyone else he'd tease, play around and drag this on, but there are impatient, desperate hands wrapped in his hair, and Senku's breath is hiccuping into sobs, and Byakuya sinks down, takes him in his mouth in a smooth, practiced move, and Senku  _ wails _ . 

"Dad, fuck,  _ please - _ " he whines, loud and desperate, his legs quivering, his hips bucking up instinctively to seek more friction; it's so obvious no one has ever touched him like this before, so evident this is so much more pleasure than he's ever felt in his entire life, and it sends a nearly unbearable thrill up Byakuya's spine, that he's the first, that he's the only one, will be the only one; he tilts his head, sucks, dives in deeper, takes all of Senku and allows him to thrust up, lets him take whatever he needs, do whatever he wants - and Senku is unravelling in the most beautiful sounds Byakuya has ever heard, whines and moans and  _ dad _ , over and over again, unmaking himself on top of Byakuya's sheets, by his hands, by his touch. 

There's no way of dragging this out, Byakuya knows. Senku is too inexperienced, too worked-up - he lasts a wonderful couple of minutes before he's coming, all noise shushed into quiet by pure heat stronger than anything he's felt before, and Byakuya is desperately greedy, swallowing it all, relishing in the bitter taste of his son's pleasure on top of his tongue. He himself comes untouched as soon as he pulls out, hips jerking, gasping hot and dripping adoration on the softness of Senku's thighs, harder than he even thought possible, so all-encompassing he thinks he might die, thinks he might pass out and never recover from this. 

When he gets some clarity of mind back together, Senku is tugging at his hair weakly, an unspoken command; Byakuya goes, lets himself be pushed against the bed once more, and Senku is lying down by his side, resting his head on top of his chest, perfect and small and  _ his _ . 

Byakuya wraps an arm around his waist, pulls him closer, presses a devoted kiss to his forehead. Senku lets out a tiny pleased noise, and they catch their breaths together like that, holding each other, pressed close and inseparable. 

"Hey," Senku says after a moment, voice slurring with tiredness. 

"What is it?"

Senku takes a deep, shuddering inhale. "Tell me you love me."

Byakuya's heart melts inside his chest. "I love you more than I could possibly explain."

Senku smiles, the softest Byakuya has ever seen from him, and says, "I love you too," and falls asleep. 

As he holds his son close to himself, as he thinks of grabbing a towel to clean both of them up and what he's going to make for breakfast tomorrow, he feels at peace, completely and perfectly. 

**Author's Note:**

> what happened in this plot is that i knew Byakuya was too protective and careful of Senku to ever do such things to him, even if he desperately wanted to - so, some sort of external influence had to happen, and Gen just...viciously nudged his way in. so, there we go.


End file.
